Using multiple instances of an aspx user control in a aspx page

I am facing a problem in using more than one instance of an aspx user control in a aspx page. This happens when I tried to fetch User control element value through Java script.

User Control Code:

<script type="text/javascript">
    function ucFun()
    {
        var a = document.getElementById("<%=tbName.ClientID%>");
        alert(a.value);
        return false;
    }
</script>
<asp:Label Text="Name" runat="server" ID="lblname"></asp:Label>
<asp:TextBox ID="tbName" runat="server" ></asp:TextBox>
<asp:Button ID="btSubmit" runat="server" Text="Go" OnClientClick="ucFun()" />

Web Form Code

<html xmlns="http://www.w3.org/1999/xhtml">
    <head runat="server">
        <title></title>
    </head>
    <body>
        <form id="form1" runat="server">
            <div>
            <uc:cont runat="server" ID="ucID" />
                <uc:cont runat="server" ID="Cont1" />
                <uc:cont runat="server" ID="Cont2" />
            </div>
        </form>
    </body>
</html>

on clicking the Go button, it displays null int he alert box, as the element is undefined. However, When I have one instance of User control in the form, it rightly displayed the text box value.

Is there any way we should write this..

Solution

After some searching found a different solution.

http://www.aspsnippets.com/Articles/Issue-JavaScript-in-WebUserControl-not-working-when-used-multiple-times-on-same-page.aspx

Here they append an uniqueID with the javascript name, thereby there is no conflict with the javascripts.

UserControl.CS

protected string uniqueKey;
protected void Page_Load(object sender, EventArgs e)
{
    this.uniqueKey = Guid.NewGuid().ToString("N");
    this.Button1.Attributes["onclick"] = "return DisplayMessage_" + uniqueKey + "();";
}

UserControl.aspx

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="UC_TestCS.ascx.cs" Inherits="UC_TestCS" %>
<script type ="text/javascript">
function DisplayMessage_<%=uniqueKey %>() {
    var message = document.getElementById("<%=TextBox1.ClientID %>").value;
    alert(message);
    return false;
}
</script>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Show Message" />